Recently, there has been widely utilized a jet propulsion craft of the small recreational propelled upon the surface of water, particularly, sea water, by means of water ejected from a jet pump accommodated within the craft.
Such a jet propulsion craft or ship comprises a main body generally composed of a bottom plate generally having a curved shape in cross section and an upper plate constituting an upper deck. An engine is mounted upon the bottom plate at substantially the middle portion thereof and a jet pump is connected to the engine so as to be driven thereby and is mounted upon the rear portion of the bottom plate.
A user, that is, a driver rides upon the rear portion of the upper plate of the craft and operates a steering handle, so as to drive the craft, which is mounted to the upper plate so as to be adjustable in height with respect thereto.
When the user drives the craft, water, preferably sea water, is introduced into the jet pump by sucking the same through a suction port formed within the ship bottom plate and the inhaled water is then exhausted rearwardly from the jet pump, whereby the craft can be propelled forwardly upon the water surface.
With the jet propulsion craft of the type described, an exhaust system is also connected to the engine so as to extend forwardly of the craft or ship and the exhaust system includes a manifold connected to an exhaust port of the engine, and an exhaust muffler connected at one end thereof to the manifold and at the other end thereof to an exhaust outlet tube disposed at the front portion of the bottom plate. The exhaust gas from the engine is exhausted from the outlet tube and the noise of the exhaust gas is eliminated in a stepwise manner during the passage thereof through the exhaust system.
With the jet propulsion craft of the type having the construction described above, however, the outlet opening of the exhaust pipe is opened at a position located above the water surface and, hence, the exhaust gas is exhausted into the atmosphere with a considerably large amount of noise.
In order to substantially decrease the exhaust gas noise below the permissible level thereof, there is also proposed a jet propulsion craft provided with an exhaust system which includes an additional muffler operatively connected to the outlet tube. The location of the additional muffler, however, renders the exhaust system complicated and occupies additional space as well as increases the manufacturing cost.
In another aspect of the prior art, there has also been proposed a jet propulsion craft such as that disclosed in the Japanese Patent Laid-open Publication No. 62-125995, in which an exhust system for an exhaust gas from an engine extends rearwardly. The exhaust gas from the engine is guided through the exhaust system into a cylindrical member in which a jet pump is accommodated. With the jet propulsion craft of this type, it is necessary to design the cylindrical member so as to ensure a large space for the purpose described above, which is not advantageous for the craft from the viewpoints of the hull construction and the manufacturing cost, and even in connection with such construction, the noise of the exhaust gas is still not sufficiently eliminated.
The prior art has further proposed a marine exhaust system for a boat as disclosed in the U.S. Pat. No. 4,002,136, in which an exhaust chamber is additionally located within the stern of the boat and is connected to the engine exhaust manifold. A submerged exhaust port emits the exhaust gas below the water level so as to silence the exhaust noise. With this exhaust system, the exhaust chamber is additionally located within the space defined between the bottom plate and the upper plate at the stern portion of the boat, which is not desired for optimum hull construction as well as the manufacturing cost thereof. Even in connection with the structure described, the noise of the exhaust gas may not be sufficiently eliminated.